fatal1ty is better in that it has X-RAM - onboard memory for sounds. it is used by a few games but not many though. It also has a connector for an external break-out box, should you ever need one.
On the other side: the Gamer (non-fatality, the cheaper one) has an optical out and a connector for a standard front audio on most PC cases.
it does not work that way. XRAM does not take over from PC RAM. If a game is written to take full advantage of EAX 5.0 then some samples are being loaded into XRAM. So if you dont have it it wont work. I am not saying it makes a lot of difference sound-wise but it does make audible difference in e.g. Q4 and I use fatal1ty card myself. If you are on budget then don't worry about XRAM though it is not THAT important as it used only in a few games.